President Trump will land in Europe next week amid fears that he will blow up a key summit focused on Europe’s defense and then offer concessions to NATO’s main adversary, Russian President Vladi­mir Putin.

The allies’ worries and Moscow’s hopes are rooted in Trump’s combative approach to foreign policy. In recent days, Trump has told senior aides that he wants to slash U.S. spending on Europe’s defense if the allies are unwilling to contribute more to NATO, a senior administration official said.

The private comments reflect a president who has shown little interest in the long history that undergirds America’s alliances or the collective foreign policy expertise of the U.S. government, according to current and former U.S. and European officials.

Instead, he relies on his instincts and his ability to forge a personal bond with world leaders. White House officials tout the president’s willingness to question long-held assumptions and challenge America’s allies — who have underspent on security for decades — to contribute more to their own defense.

But his approach has also heartened autocrats, such as Putin, who see in Trump someone willing to forgive past sins in pursuit of a deal, the officials said. And it has alarmed allies and some of Trump’s closest aides, who are concerned he may yield on issues such as Russia’s annexation of Crimea and its continuing destabilization of Ukraine.

Even as his administration has imposed tough sanctions on Moscow and expelled Russian diplomats, Trump has avoided criticizing Putin. He will meet with Putin in Helsinki on July 16.

Last year, President Trump jokes with British Foreign Minister Boris Johnson, left, NATO Secretary General Jens Stoltenberg and British Prime Minister Theresa May during a working dinner meeting at NATO headquarters in Brussels. (Matt Dunham/AP)

“The president thinks he can be friends with Putin,” former national security adviser H.R. McMaster complained during his time in the White House, according to U.S. officials. “I don’t know why, or why he would want to be.”

The president’s approach also has been corrosive to relations with allies who increasingly believe that Trump — on trade, NATO and diplomacy — is undercutting the post-World War II order in pursuit of short-term, and likely illusory, wins.

During an April visit by French President Emmanuel Macron to the White House, a frustrated Trump was sharply critical of both British Prime Minister Theresa May and German Chancellor Angela Merkel, U.S. and European officials said. Asked about his comments, the president in a statement to The Washington Post said that “immigration is destroying Europe as we know it and it is very sad to be witness to what is happening.”

European Council President Donald Tusk has derided Trump’s “capricious assertiveness” and warned that European Union countries need to prepare for “worst-case scenarios.” Trump, for his part, frequently tells European leaders how much he dislikes the E.U. — and how it is “worse than China.”

In the days leading up to the NATO summit, the president and his team have sent mixed messages. Kay Bailey Hutchison, the U.S. ambassador to the alliance, focused on the positive, touting the biggest increases in defense spending by the allies since the end of the Cold War. U.S. Ambassador to Russia Jon Huntsman pledged that Trump would “continue to hold Russia accountable for its malign activity.”

Trump by contrast has highlighted grievances. “I’m going to tell NATO — you got to start paying your bills,” he said at a rally this week in Montana. “They kill us on trade. They kill us on other things. . . . And on top of that they kill us with NATO.” And he defended Putin, calling him “fine” at the event.

This report is based on interviews with U.S. and European officials who spoke on the condition of anonymity to describe internal deliberations and Trump’s interactions with world leaders. The core of Trump’s freewheeling approach has been in place since his earliest days in the White House. Shortly after he took office, Trump began passing out his personal cellphone number to a handful of foreign leaders, and in April 2017, White House aides were startled when officials in Canada issued a standard summary of a conversation between Prime Minister Justin Trudeau and Trump. In it, Trudeau complained of “unfair duties” and “baseless” claims about trade by Trump administration officials.

No one at the White House was aware the call had taken place. “We had no idea what happened,” a senior U.S. official said.

Typically, such calls, even with close allies, are choreographed affairs. Regional experts prepare talking points covering the wide array of issues that might be raised. The national security adviser will brief the president ahead of the call and remain by his side to offer advice. After the call, a transcript is distributed to key aides, who will issue a public readout.

In this instance, U.S. officials had to rely on Trump’s memory. A terse public readout described “a very amicable call.”

After the call, White House aides urged Trump to route all conversations with foreign leaders through the Situation Room, as required under federal records law, the senior official said.

Trump’s lack of preparation has added a further level of unpredictability to his interactions with foreign leaders, the officials said. The president rarely reads his nightly briefing book, which focuses on issues likely to come up in meetings, a second senior U.S. official said. To slim down Trump’s workload, aides have sometimes put the most critical information in a red folder, the official said.

In November and again in March, Trump invited Putin to the White House for a summit against the advice of aides, who argued that the chances of progress on substantive issues was slim.

For Trump, the meeting was the point. In an interview with Fox News last month, Trump speculated that he and Putin could potentially hash out solutions to Syria and Ukraine over dinner.

“I could say: ‘Would you do me a favor? Would you get out of ­Syria,’ ” Trump said. “ ‘Would you do me a favor? Would you get out of Ukraine.’ ”

Some White House officials worry that Putin, who has held several calls with Trump, plays on the president’s inexperience and lack of detailed knowledge about issues while stoking Trump’s grievances.

The Russian president complains to Trump about “fake news” and laments that the U.S. foreign policy establishment — the “deep state,” in Putin’s words — is conspiring against them, the first senior U.S. official said.

“It’s not us,” Putin has told Trump, the official summarized. “It’s the subordinates fighting against our friendship.”

In conversations with Trudeau, May and Merkel, Trump is sometimes assertive, brash and even bullying on issues he feels strongly about, such as trade, according to senior U.S. officials. He drives the conversation and isn’t shy about cutting off the allies mid-sentence to make his point, the officials said.

With Putin, Trump takes a more conciliatory approach, often treating the Russian leader as a confidant.

“So what do you think I should do about North Korea?” he asked Putin in their November 2017 telephone call, according to U.S. officials. Some of those officials saw the request for advice as naive — a sign that Trump believes the two countries are partners in the effort to denuclearize the Korean Peninsula. Other officials described Trump’s query as a savvy effort to flatter and win over the Russian leader, whose country borders North Korea and has long been involved in diplomacy over its nuclear program.

A similar dynamic has played out in Syria, where Putin has offered to cooperate with the U.S. military on counterterrorism and help Trump realize his goal of an American withdrawal.

Trump’s more hawkish current and former advisers, including McMaster, disparaged Putin’s offer as a cynical ploy, and maintain that Russia’s primary goal is to prop up the regime of Syrian President Bashar al-Assad and, more broadly, undercut U.S. influence in the Middle East.

The Pentagon views Russia’s proposal with similar skepticism, U.S. officials said.

Ahead of the NATO summit, European officials have huddled to discuss how to avoid a repeat of the Group of Seven meeting in June, in which Trump arrived late, left early and refused to sign a customary joint statement with the other leaders.

Guiding nearly all of Trump’s interactions with world leaders is his belief that his ability to win over, charm and cajole foreign leaders is more important than policy detail or the advancement of strategic goals. Often, the calls can be discursive and confounding. In conversations with the British prime minister, he has boasted about his properties in the United Kingdom, asked her about his Cabinet officials’ performance and sometimes castigated her for being too “politically correct,” U.S. and British officials said.

Trump focused part of a meeting earlier this year with the Swedes, who are important interlocutors on North Korea, on complaints about the trade deficit, startling the visiting prime minister; the United States does not have a big trade deficit with Sweden relative to other European countries.

“Really?” Trump said last year when Irish officials visiting the Oval Office asked for a fix for undocumented immigrants from their country in the United States. “You guys? Really?”

On one point, Trump has been consistent: He rarely ends a call with a head of state without extending an invite to the White House. “Next time you’re in Washington, stop by for lunch at the White House,” he often says, according to U.S. officials.

He has made the offer when his advisers urged him not to. Such was the case with Putin and with Michel Temer, the president of Brazil, who was weighed down by corruption allegations and deeply unpopular when Trump spoke with him last fall.

Before the call, aides had urged him not to invite the Brazilian leader to the White House. Trump did it anyway. White House aides spent the next several weeks dodging calls from the Brazilian ambassador trying to set up the meeting.